In lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink the ink receptive regions accept the ink and repel the water. The ink is then transferred to the surface of suitable materials upon which the image is to be reproduced. In some instances, the ink can be first transferred to an intermediate blanket that in turn is used to transfer the ink to the surface of the materials upon which the image is to be reproduced.
Lithographic printing plate precursors useful to prepare lithographic (or offset) printing plates typically comprise one or more imageable layers applied over a hydrophilic surface of a substrate (or intermediate layers). The imageable layer(s) can comprise one or more radiation-sensitive components dispersed within a suitable binder. Following imaging, either the exposed regions or the non-exposed regions of the imageable layer(s) are removed by a suitable developer, revealing the underlying hydrophilic surface of the substrate. If the exposed regions are removed, the element is considered as positive-working. Conversely, if the non-exposed regions are removed, the element is considered as negative-working. In each instance, the regions of the imageable layer(s) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water or aqueous solutions (typically a fountain solution), and repel ink.
“Laser direct imaging” methods (LDI) have been known that directly form an offset printing plate or printing circuit board using digital data from a computer, and provide numerous advantages over the previous processes using masking photographic films. There has been considerable development in this field from more efficient lasers, improved imageable compositions and components thereof.
Various radiation-sensitive compositions are known for use in negative-working lithographic printing plate precursors as described for example in U.S. Pat. No. 6,309,792 (Hauck et al.), U.S. Pat. No. 6,893,797 (Munnelly et al.), U.S. Pat. No. 6,727,281 (Tao et al.), U.S. Pat. No. 6,899,994 (Huang et al.), and U.S. Pat. No. 7,429,445 (Munnelly et al.), U.S. Patent Application Publications 2002/0168494 (Nagata et al.), 2003/0118939 (West et al.), and EP Publications 1,079,276A2 (Lifka et al.) and 1,449,650A2 (Goto et al.).
U.S. Pat. No. 7,429,445 (Munnelly et al.) describes on-press developable negative-working lithographic printing plate precursors that contain various infrared radiation absorbing dyes that have tetraaryl pentadiene chromophores, and nonionic phosphate acrylates to increase imaging sensitivity.
U.S. Patent Application Publication 2010/0075260 (Sasaki) describes a printing plate making method using a plate precursor having a compound that generates an acid light or heat, and the imaged precursor is developed on-press and has color contrast for inspection.
In general, lithographic printing plates contain a colorant (dye or pigment) in the radiation-sensitive composition (imageable layer) that has the function of making the image visible in order to be inspected and evaluated for plate positioning and bar code readability prior to pre-press calibration. Such colorants provide contrast between the image and the background. This image contrast (print out) is particularly needed for printing plate precursors designed for development on-press. However, certain lithographic printing plates cannot contain a colorant for different reasons. For example, the imaged lithographic printing plate precursors that are usually developed on-press have a colorless coating because if a colorant is present, it could contaminate the lithographic printing ink and the fountain solution used for development and printing, with the result of altering the printed color shades. However, sometimes it is necessary for such printing plates to be used the same way as those developed off-press. In such instances, the image needs to be seen and evaluated.
U.S. Pat. No. 6,451,491 (Dhillon et al.) describes the high loading of contrast-providing pigments into the imaging layer using specific poly(vinyl acetal) polymers and specific combinations of loading solvent mixtures. Such high amounts of pigments may not be suitable as they can destabilize imaging chemistry or developers used to remove non-imaged regions in negative-working lithographic printing plate precursors.
Other contrast-providing colorants are obtained from leuco dyes that become colored in the presence of an acid or thermal acid generator, as described for example, in U.S. Pat. No. 7,402,374 (Oohashi et al.), U.S. Pat. No. 7,425,406 (Oshima et al.) and U.S. Pat. No. 7,462,440 (Yamasaki). These imaging materials have some disadvantages because the acid or radical forming mechanism can be triggered prematurely during the drying of the plate leading to un-wanted color, especially in on-press developed printing plates.
Various negative-working imageable elements have been designed for processing or development “on-press” using a fountain solution, lithographic printing ink, or both. For example, such elements are described in U.S. Patent Application Publication 2005-263021 (Mitsumoto et al.) and in U.S. Pat. No. 6,071,675 (Teng), U.S. Pat. No. 6,387,595 (Teng), U.S. Pat. No. 6,482,571 (Teng), U.S. Pat. No. 6,495,310 (Teng), U.S. Pat. No. 6,541,183 (Teng), U.S. Pat. No. 6,548,222 (Teng), U.S. Pat. No. 6,576,401 (Teng), U.S. Pat. No. 6,899,994 (Huang et al.), U.S. Pat. No. 6,902,866 (Teng), and U.S. Pat. No. 7,089,856 (Teng).
U.S. Patent Application Publications 2005/0170282 (Inno et al.), 2005/0233251 (Kakino et al.), 2003/0068575 (Yanaka), 2006/0046189 (Kunita et al.), and 2007/0072119 (Iwai et al.), and EP Publications 1,614,541 (Callant et al.), 1,736,312 (Callant et al.), and 1,754,614 (Kakino et al.) describe lithographic printing plate precursors that contain a discoloring agent or system capable of generating a color change upon exposure for providing print-out.
U.S. Patent Application Publication 2009/0047599 (Horne et al.) describes the use of specific spirolactone or spirolactam leuco dye color formers in the imageable layer of negative-working imageable elements.
U.S. Patent Application Publication 2007/0072119 (Iwai et al.) and EP 1,849,836 (Iwai et al.) describe the use of infrared radiation-sensitive cyanine dyes.
After imaging, printing plates may be inspected to make sure the desired image has been obtained. For lithographic printing plate precursors normally processed (or developed) off-press, this inspection can occur easily before mounting on the printing press. The plate manufacturer often adds a colorant to the imaging composition to facilitate this inspection.
For imaged elements that are to be developed on-press, the image is not easily identified. Adding colorant to on-press developable imaging compositions compromises plate shelf life, on-press developability, or imaging sensitivity, and the colorant may color-contaminate printing press inks. Thus, there is a need for an adequate print-out that provides visibility to the image on the printing plate before on-press development. Simply increasing imaging energy beyond that required for image durability will result in an increase in dot gain. So, the industry needs a different way to improve the print-out without causing other problems.
The invention described in U.S. Patent Application Publication 2009-0269699 (Munnelly et al.) describes the use of IR absorbable dyes to improve color contrast in on-press developable lithographic printing plate precursor.
Copending and commonly assigned U.S. Ser. No. 12/483,323 (filed Jun. 12, 2009 by Memetea, Huang, Munnelly, and Wentz) describes the application of a coloring fluid to the imaged precursor such that the optical density in the solid exposed regions is OD2 that is greater than OD1. This coloring fluid comprises a water-insoluble colorant and a solvent that is capable of swelling the solid exposed regions of the imageable layer.
Copending and commonly assigned U.S. Ser. No. 12/397,429 (filed Mar. 4, 2009 by Simpson, Baumann, Joachin, and Flugel) describes the use of a visible pigment and dye mixture for color contrast.
Copending and commonly assigned U.S. Ser. No. 12/370,625 (filed Feb. 13, 2009 by Strehmel, Baumann, and Lummel) describes the use of water-soluble visible contrast dyes.
Despite all of these improvements to provide image contrast, there remains a need for another improved means for providing contrast between the image and background of lithographic printing plates, especially those prepared from imaged negative-working lithographic printing plate precursors that are developed on-press.